1. Field of the Invention
This invention relates to a hydraulic control circuit for an actuator such as a hydraulic motor for rotating a rotary frame of a crane.
2. Description of the Prior Art
In conventional hydraulic control circuits, the operation of an actuator is generally controlled by regulating the flow of operating fluid with use of a plunger type change-over valve. This sort of change-over valve has inferior flow characteristics over the plunger stroke, adversely affected by load pressures, and it therefore has been difficult to effect delicate control of the rotational speed of the actuator.
Similar change-over valves have also been used in hydraulic control circuits for controlling rotation of hydraulic cranes and changing the direction and speed of rotation of a hydraulic drive motor by the change-over valve. In some cases, the control circuit is provided with a pressure compensating control valve for preventing the above-mentioned variations in the flow characteristics due to load pressures. However, the provision of such a control valve neither changes the flow characteristics themselves nor allows for delicate control of the rotational speed.
There have been proposed various other control circuits incorporating a brake valve, a relief valve, a pressure control valve or a flow control valve for preventing cavitation, shocks or overloading in rotational operations. The incorporation of these valves is, due to an increased number of component parts, reflected by a complicated circuit arrangement and a high production cost and on the contrary impairs the performance quality of the rotating body.
Particularly, as compared with the rotation by mechanical drive, it is difficult in hydraulic cranes to control the rotational speed of the hydraulic motor for the reasons stated above and due to variations in speed accruing from fluctuations in engine rotation, experiencing shocks at the time of starting or stopping the motor. Another problem experienced in hydraulic cranes is difficulty in controlling the braking force.